The development of commercially acceptable coated plastic films for printing applications is often a compromise between a variety of desired properties. For example, printed labels, such as those used for beverage containers or health and beauty containers, should be capable of exposure to any severe conditions encountered during manufacturing, transport, and storage. Not only should printable coatings exhibit hot-water resistance, organic-solvent resistance, abrasion resistance, and haze resistance on exposure to hot or cold water, the coating should also exhibit good ink adhesion immediately after printing. For example, the ink on an imaged beverage container label should stay adhered to the coated film after the label is made and applied to the container, even when exposed to hot or cold water or any other subsequent abrasion that may be encountered in mechanized handling.
The combination of the ink, substrate, and printing method used greatly affects the image quality of the final printed article. For example, in contact printing methods such as screen-printing, a blade forces the ink to advance and wet the receiving substrate, while in the case of non-contact printing methods, such as inkjet printing, the individual ink drops are merely deposited on the surface. Accordingly, ink/substrate combinations that result in good image quality when printed with contact methods such as screen printing, often exhibit insufficient wetting when printed with non-contact printing methods such as inkjet printing resulting in low radial diffusion of the individual ink drops on the surface of the substrate (e.g., “dot gain”), low color density, and banding effects (e.g., gaps between rows of drops).
Inks used in the various printing methods often have different physical properties. For example, inks used in screen printing and lithography printing techniques typically do not meet the low viscosity requirements of inks used in inkjet printers. Screen printing ink compositions typically have a viscosity of at least two orders of magnitude greater than the viscosity of inkjet printing inks, and it is not generally feasible to dilute a screen printing ink to make it suitable for inkjet printing as the addition of large amounts of low viscosity diluents drastically deteriorates the ink performance and properties, particularly the durability. Additionally, inkjet inks may vary depending on the type of inkjet printing used. For example, some inkjet printers use water or solvent based inks, while UV-inkjet printers generally use solvent free (i.e., 100% solids) inks.
It would be highly desirable to have a plastic film that is suitable for use in inkjet printing applications, particularly those using UV-curable inkjet ink. It would be desirable to have a coated film that has good ink adhesion immediately after printing, especially at high printing speeds and allows for the formation of high quality print images.
U.S. Pat. No. 6,596,379 discloses coating compositions and plastic films thus coated. The coating composition comprises a cationically stabilized emulsion polymer comprising a combination of at least one polymerizable monomer which is uncharged or positively charged in an aqueous solution having a pH between 1 and 8, polymerized in the presence of at least one water-soluble polymer having a number-average molecular weight greater than 5000 which comprises a moiety selected from the group consisting of primary amines, secondary amines, tertiary amines, and quaternary ammonium salts.
U.S. Pat. No. 6,893,722 discloses a cationically stabilizable amino-functional polymer having a number-average molecular weight greater than 3000 which exists in the presence of water as a solution or stable emulsion only when the pH is less than or equal to 8. Upon drying the polymer contains ethenically unsaturated moieties selected from the group consisting of acrylic, methacrylic, and enamine. The polymer is useful for promoting adhesion of curable inks and other coatings for plastic films.